Heads-up displays allow a user to view a scene while relevant information is overlaid on the scene, so that the user looking through the heads-up display simultaneously sees both the scene and the relevant information. For example, a pilot looking through a heads-up display while landing an airplane simultaneously sees the airport ahead (the scene) through the heads-up display while the heads-up display projects information such as speed, heading and altitude (the relevant information) that the pilot needs to land the plane.
One problem associated with heads-up displays is how to direct an image from a display to the user's eye with reasonable field of view (FOV) and eyebox. In displays with a large FOV and eyebox, the thickness of the optics quickly become on the order of a few centimeters instead of a few millimeters, making it difficult or impossible to integrate them in sleek, lightweight form factors such as a monocle or eyeglasses.